POS0847 MYOSITIS AUTOANTIBODIES DETECTED BY LINE BLOT IMMUNOASSAY: CLINICAL ASSOCIATIONS AND CORRELATION WITH ANTIBODY TITERS
Authors: J. Loarce-Martos, L. Calvo Sanz, S. Garrote-Corral, C. Rits, R. Ballester Gonzalez, A. García-Soidan, J. Bachiller-Corral, G. Roy
Journal: Annals of the Rheumatic Diseases (2022)

Background Idiopathic inflammatory myopathies (IIM) are a heterogeneous group of autoimmune diseases (AID) characterized by muscle inflammation and weakness, often accompanied with other organ involvement, such as skin rash or interstitial lung disease (ILD). Myositis specific (MSA) and myositis associated antibodies (MAA) can be detected in approximately 60% of patients with IIM. Besides, antibody titers have been suggested to be related with diagnostic accuracy, although it has not been widely studied. MSA are considered to be exclusive of patients with IIM, whilst MAA can occur in IIM and other systemic autoimmune diseases, nevertheless, most of the studies are focused exclusively on IIM patients. Objectives The aim of this study is to assess the relationship between MSA/MAA and diagnosis (including IIM and other AID), and to explore the impact of antibody titers in diagnostic accuracy. Methods We retrospectively reviewed all the serum samples obtained from patients tested for MSA/MAA between 01/01/2018 and 31/12/2020 in the Immunology department of Ramón y Cajal University Hospital (Spain). These antibodies were tested by line blot immunoassay (LIA) (EUROLINE Autoimmune Inflammatory Myopathies 16 Ag, Euroimmun, Lübeck, Germany). Positivity was stablished according to absorbance titer and adjusted by positive control of each test (arbitrary units, AU). True positive (TP) MSA and MAA were defined as those patients with IIM or AID with phenotypes expected of that MSA/MAA, according to the available information. The patients that did not have a phenotype compatible with that antibody were regarded as false positive (FP). Statistical analysis was carried out using IBM SPSS statistics version 22. Results We analyzed 130 positive samples which corresponded to 130 patients, 85 were women and mean age was 55.08 years. 44 patients (33.8%) were classified as IIM, 43 (33.1%) as AID, and 43 (33.1%) as non-IIM/AID. Among these 130 patients, 164 MSA/MAA were detected. 83 (50.6%) positive MSA/MAA were regarded as TP, and 81 (49.4%) as FP (positive predictive value [PPV] 50.6%). Antibodies regarded as TP had a higher antibody titer compared to FP (49,19 AU vs 26,96 AU, p&lt;0.001). This difference was statistically significant for MSA and MAA when analysed separately (Figure 1). FP antibodies were associated with negative ANA and low titer ANA (p&lt;0.001). Multiple positive antibodies (antibodies included in samples that were positive for &gt; 1 MSA/MAA) were more frequently FP in comparison with isolated positive MSA/MAA (p&lt;0.001). Figure 1. Autoantibody titers comparison between false positives and true positives. MSA=myositis specific antibody, MAA=myositis associated antibody, FP=false positive, TP=true positive Conclusion In this study we confirm that FP results using LIA are relatively frequent, and are associated with lower titer MSA/MAA, negative ANA, lower titer ANA, and with multiple positive MSA/MAA within one sample. References [1]Betteridge Z, Tansley S, Shaddick G, et al (2019) Frequency, mutual exclusivity and clinical associations of myositis autoantibodies in a combined European cohort of idiopathic inflammatory myopathy patients. J Autoimmun 101:48–55. https://doi.org/10.1016/j.jaut.2019.04.001 [2]Cavazzana I, Fredi M, Ceribelli A, et al (2016) Testing for myositis specific autoantibodies: Comparison between line blot and immunoprecipitation assays in 57 myositis sera. J Immunol Methods 433:1–5. https://doi.org/10.1016/j.jim.2016.02.017 [3]Tansley SL, Li D, Betteridge ZE, McHugh NJ (2020) The reliability of immunoassays to detect autoantibodies in patients with myositis is dependent on autoantibody specificity. Rheumatol (United Kingdom) 59:2109–2114. https://doi.org/10.1093/rheumatology/keaa021 [4]Mahler M, Betteridge Z, Bentow C, et al (2019) Comparison of Three Immunoassays for the Detection of Myositis Specific Antibodies. Front Immunol 10:848. https://doi.org/10.3389/fimmu.2019.00848 Disclosure of Interests None declared